Many workers in the art of catalysis have centered their attention on efforts to create gold catalysts of a fine size effective to catalyze the oxidation of carbon monoxide to carbon dioxide, particularly in the presence of a large excess of hydrogen and in mixtures of gases wherein the carbon monoxide is a small percentage. The fine size thought to be beneficial for providing a large contact surface for the reactants. See, for example, Haruta et al U.S. Pat. Nos. 4,839,327 and 5,506,273. Some workers deposit and therefore spread out their catalyst compositions on porous supports, such as described by Grigorova et al in U.S. Pat. Nos. 5,665,668 and 5,759,949; Haruta et al, in U.S. Pat. No. 4,939,219, propose placing the gold on an alkaline earth metal support.
In German patent DE 198 36 585 C1, Plzak first states that it is desirable for the catalyst materials of his invention to have specific surface areas as high as possible and then describes three variations of his preparation of gold-containing catalysts for CO oxidation. In the first, the material is produced sequentially—that is, an iron salt is first reacted with a base to form an iron hydroxide gel, then the gel is impregnated with a solution of a gold compound, to “deposit complexed Au clusters on the surface of the hydroxide gel in the finest distribution” (um komplexierte Au-Cluster auf der Oberfläche des Hydroxidgels in feinster Verteilung abzuscheiden—p3, line 44. The authors go on to say that much smaller Au clusters can be fixed on the Fe2O3 support material by the sequential process than by a coprecipitation process. The sequential process is therefore considered to be superior to the coprecipitation process, but Plzak does not recognize the disadvantages of using a powder in the CO oxidizing step.
In Plzak's second stated process, a sinter inhibitor which is an oxide of aluminum, chromium, or magnesium is created during the first step of iron precipitation by including with the reactants a water soluble salt of Al, Cr or Mg. A third variation calls for the inclusion of the Al, Cr or Mg salt along with all the other reactants in the coprecipitation procedure. The sequential process is still considered by Plzak to be superior.
The reader may be interested in Haruta et al's Example 1 of U.S. Pat. No. 5,506,273 which prepares a catalyst containing a metal oxide and gold, said to be useful for oxidation of carbon monoxide.
The art is in need of a convenient process for oxidizing CO in a mixture including a high level of hydrogen with an acceptable pressure drop.